In various industrial fields, various efforts are underway on a worldwide scale to reduce environmental impacts and environmental loads. Especially in the field of automobile industry, the widespread use of not only gasoline engine vehicles with high fuel efficiency performance but also so-called eco-friendly vehicles such as hybrid vehicles and electric automobiles is promoted, and the development focused on further enhancement of the performance of such vehicles is advanced day by day.
An exhaust system for exhaust gas, which connects a vehicle engine to a muffler, is usually provided with a catalytic converter for cleaning exhaust gas.
An engine may emit substances harmful to the environment, such as CO, NOx, and unburned HC and VOC. In order to convert such toxic substances into acceptable substances, a catalytic layer made of a noble metal catalyst such as palladium or platinum is formed on each of cell wall surfaces of a large number of cells included in a substrate. More specifically, on each of the cell wall surfaces of a large number of the cells, the catalytic layer is formed to extend along a longitudinal direction of the substrate, which is a direction along which exhaust gas flows. As the exhaust gas passes through a catalytic converter including such a substrate configured as described above, CO is converted into CO2, NOx is converted into N2 and O2, and VOC is burned to produce CO2 and H2O.
In commonly-used catalytic converters, the cell density in a substrate having, for example, a honeycomb structure is uniform. Such catalytic converters have a problem that it is difficult to make full use of catalytic layers of the whole substrate because the exhaust gas flow velocity is higher in a central region in a cross-section of the substrate than in a peripheral region in the cross-section of the substrate. If a catalytic converter in which the cell density is higher in a central region of a substrate than in a peripheral region of the substrate is provided in consideration of the exhaust gas flow velocity distribution described above, it is possible to minimize a variation of the flow velocity distribution within a cross-section of the substrate. This makes it possible to effectively utilize catalytic layers of the whole catalytic converter to clean exhaust gas.
Patent Literature 1 discloses a technique for improving the exhaust gas cleaning performance. According to this technique, in a substrate of a catalytic converter (a catalyst body, in this case) having, for example, a honeycomb structure in which the cell density is uniform as a whole, the amount of noble metal catalysts supported on a central region (a central portion, in this case) of the substrate is made different from the amount of noble metal catalysts supported on a peripheral region (an outer peripheral portion, in this case) thereof. More specifically, in the catalyst body, the amount of catalysts per unit volume, supported on the central portion where a large amount of gas flows, is at least 1.1 times as large as the amount of catalysts per unit volume, supported on the outer peripheral portion. However, in this technique, the cell density is uniform in the whole catalyst, and thus high exhaust gas cleaning performance cannot be expected.